In a cellular communication system, each of the subscriber units (typically mobile stations) communicates with typically a fixed base station. Communication from the subscriber unit to the base station is known as uplink and communication from the base station to the subscriber unit is known as downlink. The total coverage area of the system is divided into a number of separate cells, each predominantly covered by a single base station. The cells are typically geographically distinct with an overlapping coverage area with neighbouring cells. FIG. 1 illustrates a cellular communication system 100. In the system, abase station 101 communicates with a number of subscriber units 103 over radio channels 105. In the cellular system, the base station 101 covers users within a certain geographical area 107, whereas other geographical areas 109, 111 are covered by other base stations 113, 115. Some overlap areas 117 can be covered by more than one cell.
As a subscriber unit moves from the coverage area of one cell to the coverage area of another cell, the communication link will change from being between the subscriber unit and the base station of the first cell, to being between the subscriber unit and the base station of the second cell. This is known as a handover. Specifically, some cells may lie completely within the coverage of other larger cells.
All base stations are interconnected by a fixed network. This fixed network comprises communication lines, switches, interfaces to other communication networks and various controllers required for operating the network. A call from a subscriber unit is routed through the fixed network to the destination specific for this call. If the call is between two subscriber units of the same communication system the call will be routed through the fixed network to the base station of the cell in which the other subscriber unit currently is. A connection is thus established between the two serving cells through the fixed network. Alternatively, if the call is between a subscriber unit and a telephone connected to the Public Switched Telephone Network (PSTN) the call is routed from the serving base station to the interface between the cellular mobile communication system and the PSTN. It is then routed from the interface to the telephone by the PSTN.
A cellular mobile communication system is allocated a frequency spectrum for the radio communication between the subscriber units and the base stations. This spectrum must be shared between all subscriber units simultaneously using the system.
One method of sharing this spectrum is by a technique known as Code Division Multiple Access (CDMA). In a Direct Sequence CDMA (DS-CDMA) communication system, the signals are prior to being transmitted multiplied by a high rate code whereby the signal is spread over a larger frequency spectrum. A narrowband signal is thus spread and transmitted as a wideband signal. At the receiver, the original narrowband signal is regenerated by multiplication of the received signal with the same code. A signal spread by use of a different code will not be de-spread by the receiver but will remain a wide band signal and removed by filtering after the de-spreading operation. In the receiver, the majority of interference caused by interfering signals received in the same frequency spectrum as the wanted signal can thus be removed by filtering. Consequently, a plurality of subscriber units can be accommodated in the same wideband spectrum by allocating different codes for different subscriber units. Codes are chosen to minimise the interference caused between subscriber units typically by choosing orthogonal codes when possible. A further description of CDMA communication systems can be found in ‘Spread Spectrum CDMA Systems for Wireless Communications’, Glisic & Vucetic, Artech house Publishers, 1997, ISBN 0-89006-858-5. Examples of CDMA cellular communication systems are IS 95 standardised in North America and the Universal Mobile Telecommunication System (UMTS) currently under standardisation in Europe.
Each base station transmits a pilot signal, which can be received by the subscriber units. A subscriber unit measures the pilot signal level of the transmitted pilot signal from a number of surrounding base stations. Each base station furthermore transmits information of neighbouring base stations enabling the subscriber unit to search for the pilot signals of these cells. The subscriber unit reports the measured signal values back to the network and the preferred serving cell or cells is chosen based at least partly on these levels. In the simplest form, the serving cell is chosen as the cell whose pilot signal is received at the highest level, as this will maximise the quality of the transmissions between the subscriber unit and the base station with the least cost in terms of power. In this case, a subscriber unit served by another cell will be handed over to this cell. In a real communication system, such as UMTS, more complex algorithms are used which may take into account factors such as the bit error rate of an ongoing communication, the time (handover margin) etc. However, the measured pilot signal strength is one of the most important parameters considered in determining a serving cell (or cells) whether for hand over or for system access.
Spread spectrum systems offer high capacity by allowing a frequency reuse factor of one. This means that each cell transmits on the same frequency. While, this allows maximum usage of the radio spectrum, it causes each cell to be an interference source to every other cell within its transmission range. In an interference limited spread spectrum system the major loss of available capacity is multiple pilot interference. This is the case where not only the serving cell and its adjacent neighbour provide coverage to a mobile between these cells, but other unwanted cells also degrade the area with their signals. Normally, this is taken care of by drive testing and making small incremental changes in order to try to reduce these multiple pilot regions. However, this is resource intensive and requires skilled personnel. A second method of reducing these multiple pilot regions is based on simulations during the planning stages. However, this solution is impaired by the lack of accurate path-loss propagation tools as well as simulation assumptions of operation performance, traffic distribution e.t.c., which do not track mobile performance in the field very well.
There is thus a need for an improved system for setting of pilot signals transmitted by base stations in a cellular communication network.